Basics of Filtration

There are at least seven mechanisms by which a filter can capture particles. All of these mechanisms are at work in a filter at any given time to varying degrees and may change as operating conditions change. The seven mechanisms of particle capture are listed below:

Direct Interception

Direct interception is usually the governing mechanism in liquid filtration. Interception of a particle occurs by this method when a particle approaches a media obstruction a distance equal to or less than the particle radius. In essence, if the particle "runs into" a physical barrier, it becomes captured.

Bridging

One single particle may be too small to be directly intercepted or blocked by the filter medium. However, two particles hitting the obstruction at the same time may stick together and be deposited. Particles form a bridge across a pore by hitting the pore simultaneously, or by adhering to each other earlier in the process and then becoming deposited. Bridged particles may not clog the opening completely, thus creating a smaller pore that is more difficult to pass through. The gradual accumulation of particles on the filter medium is known as the formation of a filter cake. This cake creates a finer matrix for subsequent interception.

Inertial Impaction

Similar to bridging, sieving is a specialized case of direct interception. Sieving occurs when the opening or pore in the medium is more constrictive than the diameter of the particle. The particle is simply too large to pass through the pore. Sieving may occur on the surface of the filter or through the pore. Sieving may occur on the surface of the filter or throughout the depth of the medium.

Inertial Impaction

Inertial impaction is based on the scientific principle of inertia, stating that a moving object will continue to move in a straight line unless acted on by an outside force. As particles flow through a filter, they may encounter an obstruction and become captured while the fluid flows around the barrier. Due to the inertia of the particle, it continues to move in a straight line and becomes impacted. Fluid viscosity also greatly affects inertial impaction.

Fluids that are highly viscous exert greater drag on particles, reducing the chances of inertial impaction. Gases, on the other hand, have extremely low viscosity, enhancing inertial impaction to the point of being a primary mechanism of capture in gas filtration.

Diffusion Interception

The mechanism of diffusion interception is attributable to the fact that molecules are in constant random motion. This motion enhances the opportunity for a particle to become intercepted by the filter medium.

Diffusion interception is more prevalent in particles that are 0.1 to 0.3 microns in size, since small particles are most affected by molecular bombardment. Diffusion interception is primarily found in gases due to their inherently low viscosity and high degree of molecular mobility.

Electro kinetic Effects

Electrical charges may be present on the filter medium and / or on the particles. Particle deposition can occur due to attractive forces between charges or induced forces due to the proximity of the particle to the medium. Some manufacturers purposely alter the surface of the filter medium to enhance electro kinetic capture.

Gravitational Settling

Particles have mass and are therefore affected by gravity. It is possible that a particle may leave the fluid streamlines and settle in the same fashion as sediment in a settling tank. Particles may be deposited within a filter medium or in the up-stream chamber of filter housing.